Load Calculation Backup Generator

I'm thinking about buying an 8000W, 240V, portable, backup generator to connect to my house when the power goes out. This type of generator, obviously, is designed to be used with extension cords that would be routed through an open window or door, for example. However, I would like to connect it directly to the service entrance on the back of my house. I am aware that this type of connection requires a "transfer switch".

My question is, do I have to worry about how much of a load I have on each phase of the generator's output? Or, do I only have to worry about total wattage?

To clarify, let's take an extreme (and unrealistic) example. Let's say I have a house that draws 6000W on one phase and 1000W on the other phase and I connect to the service entrance via the generator's 240V,

30A receptacle. So, in this situation I would be drawing about 50 Amps on phase 1 and 8 Amps on phase 2, for a total of 7000W.

Just as an example, there is an 8000 W generator at:

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There is a schematic (PDF, page 2) for this generator at:

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This schematic shows a "120/240V 30Amp" socket connector protected by a "30Amp 2P" circuit breaker (CB1). So, I guess I've answered my own question. It looks to me like a current greater than 30A on either pole will trip the breaker.

So, it's back to the drawing board and here's where I'm definitely getting in over my head. Is it possible to parallel the windings somehow so that the two legs are in phase?

If you could do that then the total output could be connected to both bus bars in the circuit breaker box. Now in the (hypothetical) situation where one bus bar draws 6000W and the other draws 1000W, there wouldn't be a problem.

The disadvantage would be that you couldn't operate any 240V appliances. However, with these relatively small generators, you probably wouldn't want to do that anyway. A clothes dryer probably draws 5000-6000 watts all by itself. You would lose the option to operate one or two burners on your electric range, but you could use your microwave oven or your toaster oven instead.

Reply to
jaywitkow
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You are really good at answering your own questions! You are beginning to understand the tradeoffs involved in a standby power system. If you are willing to do a little intelligent load management, and perhaps give up a few of your largest loads entirely, you will do fine.

Now how are you going to fuel this monster?

Vaughn

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Reply to
Vaughn Simon

You've found the wonders of load management. You have a generator that has much lower capacity than your normal service so you have to selectively apply loads to stay within it's capacity.

When you have a power failure you turn off all the circuit breakers in your well labeled (it is well labeled isn't it?) service panel first, then connect and start your generator and then throw the transfer switch to provide generator power to the panel. After that you have to actively manage which circuits you have on.

For instance you probably want to switch on your lighting circuits, but make sure you don't have an excessive number of lights on. This is your small baseline load that you'll keep on. This load probably represents about 5A/leg on your well balanced service panel (it is well balanced isn't it?).

After your baseline load you have a variety of loads that the generator can power individually, but not together. Fortunately these loads are rarely time critical so one can "wait" while another is using the generator capacity.

Examples are:

Heat or A/C - These loads can easily stand to wait even an hour while the generator powers other loads.

Refrigerator - A refrigerator can easily go a couple hours (with minimal opening) without an issue.

Well pump - The pressure tank usually holds at least 20 gal of useable water so you can wash hands and flush toilets a few times before you have to provide power to the well pump.

Stove / oven / microwave - Again you can hold off cooking dinner until the heat or A/C has completed it's cycle and those loads can wait the hour until dinner is cooked.

Water heater - A 50gal+ well insulated water heater will maintain a reasonable water temp without power for a good while.

Of course managing these loads takes a bit of work. In really big installations automated load management is fairly common and there is no reason that such systems can't be scaled and applied to a residential sized installation.

Pete C.

Reply to
Pete C.

You can also download Homer from NREL. I use it for my cabin.

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Max
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Reply to
beemerwacker

Exactly how our friends in Punta Gorda Florida (Charlie) referred to it. "who is going to feed the monster"? It is not an insignificant amount of fuel either. They were filling up cans every day.

Reply to
gfretwell

I have used an 8000W backup generator during power outages for the house for over 20 years. It was wired into the main panel with a dedicated plug. I strongly recommend an automatic transfer switch, even though I didn't use one.

It's really no problem, once you understand your energy "budget." All the lighting was OK, but we turned off each light as we left the room. The generator ran the furnace, the freezers, fridge and TV without concern. Using the microwave, dishwasher or coffeemaker, required a bit of planning, but worked quite well if you didn't use more than one of them at a time.

We refrained from using the dryer or anything with big electric elements whenever possible. I suspect we could have used them, by turning other things off, but the outages were generally short enough that we didn't need them. The 220V submersible pump in the well was a bit of a concern, but we could generally assume it wasn't running when no one was using water. You could also tell a lot by the sound of load on the engine. The general rule was "one thing at a time, and nothing unnecessary" and everything worked fine.

I use a 10,000 watt generator now, but the same general rules apply.

Reply to
JoeSP

BIG WARNING:***************

You MUST rember to shut off the main breaker before the generator is plugged into the house! If the current from the generator flows backward into the powercompany's transformer it will be a sight to see! You may fry a lineman, or blow up the transformer.

Transformers take large voltages and reduce the voltages through differences in windings. for example power goes from big(1k+ volts) to small (115v leg's). They also go in reverse, turning small to big if used improperly!

Be careful!

--Alex

*** end big warning ***

JoeSP wrote:

Reply to
netster403

While there is a theoretical issue with safety for a lineman, consider that if the power outage is widespread and the OP fails to open the main breaker, he will simply be trying to power the whole grid with his 8 KW. Talk about "load sharing".

He won't be burning up any power company transformers but his APU won't be happy. Of course an automatic transfer switch is in order.

Reply to
Wes Stewart

It wasn't very theoretical for the families of these linesmen:

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Reply to
JoeSP

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I suspect that the line was down between the house and the local HT transformer, when the lineman picked it up to reconnect it, assuming it would be dead....

I've come across one bright spark who wired the generator to a three pin plug ( yes, with bare pins..) and plugged that in with the house isolator closed. It tripped out the genny. He re-started the genny and then went to reclose the genny trip - but, meanwhile, mains supply had been restored. ..

It just tripped out the genny again.

Genny designers do seem to know what they are doing....

Reply to
Palindr☻me

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That's utterly ridiculous. It's a well-known fact that generators can backfeed through a transformer and re-energize the line. Lines charged with kilovolts of electricity require very little current to kill someone who touches it.

Reply to
JoeSP

Suspect what you wish. Whether a dirty, sweaty lineman gets hit by 120v ac or 1200v, under many circumstances she or he'll be just as dead.

So this is not an abstract game.

J
Reply to
barry

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That's why code in most places requires an approved transfer switch to be installed. Here's one available at Home Despot:

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Best regards, Spehro Pefhany

Reply to
Spehro Pefhany

Far from being ridulous, it is one possible scenario. And, generally speaking, the same current is needed to kill someone whether delivered at 120v or 11kV - assuming a series element that limits the current to that value.

Don't US linemen either work using live circuit protocols, or tie the circuits to ground before working on them? The risk of some idiot energising what should be a dead circuit seems so obvious that I find it very hard to believe that they don't..

Reply to
Palindr☻me

After the hurricanes here in Central Florida they were driving an area to map out all the downed lines and lines with tree branches stuck in them in a section. They generally worked by splicing the wires that were low enough to touch on the supply side poles first, then when it was safe they energized that section. Finally, they spliced the rest of the downed lines hot. The crew doing restoration in my area was from Texas, and didn't have the right kind of fuses for the 7200 volt line feeding about 100 homes.

They were in fully insulated bucket trucks with HV rated gloves during all of this. Otherwise they would have had to shut down the whole area at the substation and do all the repairs which took weeks. Some power poles with minor damage weren't replaced for over a year. They had red flags and a job number nailed to them to warn workers that they were not safe to work on with anything smaller than a line bucket.

I believe we had more fatalities from traffic accidents than electrocutions. One crew's bucket truck was hit on the highway after they had finished their work, and they were headed back to their home state. :(

Reply to
Michael A. Terrell

Zero facts in those references:

I didn't mean to infer that backfeed isn't an issue, I was only pointing out the nonsense about blowing up transformers, etc.

You also removed the last line in my post where I clearly stated that a transfer switch was called for.

I stand by my statement that if the whole grid is down, an 8 KW generator isn't going to energize it or blow anything up.

This reference says about the same thing:

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"...The hazard of generator back feed seems to be greatest when lineworkers are working on a single transformer or a small section of line. "Portable generators and motors are usually not powerful enough to supply large utility loads, and the voltage will tend to collapse under those circumstances," explains Drew...."

They go on to say the obvious, "Always consider the possibility of back feed when working on de-energized lines, and consider from which direction the power could flow. "This should be discussed during each job briefing session," Drew emphasizes...."

Reply to
Wes Stewart

Anything to win an argument? Even something so stupid as not having to worry about your generator backfeeding through the lines?

Reply to
JoeSP

...

So, what you need to do is "load balance": inspect the house wiring, and find out just what the load actually is on each leg. (with USA 240VCT, they're not phases, they're legs, since there is nothing to shift the phase, it's only a center-tapped 240V).

If the house was wired by competent people, most of this should already have been taken care of, since the power company likes balanced loads too.

If you need to run the toaster oven and space heater simultaneously, for example, plug one into leg "A" (black/white) and the other into leg "B" (red/white). These can even both be on a duplex outlet, if you break the shorting tab.

Or, you could go full-goose bozo, and buy all 240V appliances. ;-)

And, just FYI, the power company should be very helpful with the crossover switch issue.

Have Fun! Rich

Reply to
Rich Grise

In the US, Linemen DO follow HOT LINE Protocol when working with ALL Repair type operations. That being Said, when they first come to a WorkSite, dealing with downed lines is a BIG DEAL, and having some DUFUS, backfeeding the grid with his toy genset, is a BIG Concern, and it happens much more often that anyone suspects.

Me been there, had to deal with that......

Reply to
Me

The thing to remember is that your Genset when in 120/240Vac mode can only supply 1/2 the total load amperage to each leg. So if you have a 6Kw genset then only 3Kw per leg can be supplied without overcurrenting that sator winding. For figuring sake, you can probubly allow an extra

10% to that if you have good airflow thru the generator core, but even that maybe iffy. If you were to use a Transfer switch, have a 120Vac only mode on the genend, and wire the two 120Vac legs together on the Genset side of the transfer switch, then you could use all 6Kw @120Vac, but none of your 240Vac Loads would operate. Most of the 240Vac Loads wouldn't really be nessesary in Emergency Powered situations anyway.

Bruce in alaska

Reply to
Bruce in Alaska

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